Fabrication of hard alloys into complex-shaped parts is often difficult since brittleness is commonly associated with hardness. For example, a cobalt-based alloy containing 29 weight % chromium, 4.5 weight % tungsten, 1 weight % carbon, and cobalt balance has a hardness of about HRC 42 (Rockwell C scale) but its room temperature tensile elongation is less than 1%. Such an alloy is difficult to form without cracking. It is usually used as castings or consumables for hardfacing, which often involves welding.
Certain materials do not lend themselves to welding due to metallurgical incompatibility. For example, titanium and its alloys cannot be hardfaced by weld overlaying with a cobalt-based or nickel-based hardfacing consumable since titanium tends to react with these alloys to form brittle intermetallic compounds, resulting in disbonding. Another similar example is aluminum and its alloys. Copper and its alloys are very difficult to weld on the surface due to their high heat conductivity. For polymeric materials, it is impossible to weld with metals. For wear protection, thermal spraying is often used.
Thermal spraying is capable of coating metals and polymeric materials with hard coatings. However, the coating thickness is often limited to less than 0.5 mm. Moreover, the properties of the coatings are significantly different from the original alloys since high amounts of embedded oxides are usually formed during the spraying process. Another weakness of any thermal spraying process is that it is a “line of sight” process. For example, a tube with a small ID cannot be coated by using thermal spraying methods.
Other coating methods, such as, physical vapor deposition (PVD), chemical vapor deposition (CVD) and electroplating, although not “line of sight”, are limited in coating material selections and thickness.